Network traffic annotation added to gaia_auth_fetcher.

Network traffic annotation is added to network request of:
google_apis/gaia/gaia_auth_fetcher.h
google_apis/gaia/gaia_auth_fetcher.cc
google_apis/gaia/gaia_auth_fetcher_unittest.cc
ios/chrome/browser/signin/gaia_auth_fetcher_ios.h
ios/chrome/browser/signin/gaia_auth_fetcher_ios.mm

BUG=656607

Review-Url: https://codereview.chromium.org/2872253002
Cr-Original-Commit-Position: refs/heads/master@{#475351}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 0d460449a6b80930bbddb2c54ca0340c8ea4bf7b
3 files changed